Semiconductor light emitting devices with densely packed phosphor layer at light emitting surface

ABSTRACT

An LED includes a chip having a light emitting surface, and a coating of phosphor-containing material on the light emitting surface. Phosphor particles are arranged in a densely packed layer within the coating at the light emitting surface, and such that the light emitting surface is in contacting relationship with the phosphor particles.

FIELD OF THE INVENTION

This invention relates to lighting devices, and more particularly tosemiconductor light emitting devices including wavelength conversionmaterials.

BACKGROUND

Light emitting diodes and laser diodes are well known solid stateelectronic devices capable of generating light upon application of asufficient voltage. Light emitting diodes and laser diodes may begenerally referred to as light emitting devices (“LEDs”). Light emittingdevices generally include a p-n junction formed in an epitaxial layergrown on a substrate such as sapphire, silicon, silicon carbide, galliumarsenide and the like. The wavelength distribution of the lightgenerated by the LED generally depends on the material from which thep-n junction is fabricated and the structure of the thin epitaxiallayers that make up the active region of the device.

Typically, an LED chip includes a substrate, an n-type epitaxial regionformed on the substrate and a p-type epitaxial region formed on then-type epitaxial region (or vice-versa). In order to facilitate theapplication of a voltage to the device, an anode ohmic contact is formedon a p-type region of the device (typically, an exposed p-type epitaxiallayer) and a cathode ohmic contact is formed on an n-type region of thedevice (such as the substrate or an exposed n-type epitaxial layer). Inother embodiments, a substrate need not be included.

It is known to enclose an LED chip in a package that can perform anumber of functions and provide a number of benefits. For example, anLED package can provide mechanical support and environmental protectionfor the chip, as well as providing electrical leads for connecting thechip to an external circuit, and heatsinks for efficient heat extractionfrom the chip. An LED package can also perform optical functions. Forexample, an LED package can include optical materials and/or structures,such as lenses, reflectors, light scattering layers, etc., that candirect light output by the semiconductor chip in a desired manner.

In a typical LED package 10 illustrated in FIG. 1, an LED chip 12 ismounted on a reflective cup 13 by means of a solder bond or conductiveepoxy. One or more wirebonds 11 connect the ohmic contacts of the LEDchip 12 to leads 15A and/or 15B, which may be attached to or integralwith the reflective cup 13. The reflective cup 13 may be filled with anencapsulant material 16 containing a wavelength conversion material suchas phosphor particles. The entire assembly may then be encapsulated in aclear protective resin 14, which may be molded in the shape of a lens tocollimate the light emitted from the LED chip 12.

It is often desirable to incorporate phosphor into an LED package toenhance the emitted radiation in a particular wavelength and/or toconvert at least some of the radiation to another wavelength. Ingeneral, light is emitted by a phosphor when a photon having energyhigher than a bandgap of the phosphor material passes through thephosphor and is absorbed. When the photon is absorbed, an electroniccarrier in the phosphor is stimulated from a resting state to an excitedstate. When the electronic carrier decays back to a resting state, aphoton can be emitted by the phosphor. However, the emitted photon mayhave an energy that is less than the energy of the absorbed photon.Thus, the emitted photon may have a wavelength that is longer than theabsorbed photon.

The term “phosphor” is used herein to refer to any materials that absorblight at one wavelength and re-emit light at a different wavelength,regardless of the delay between absorption and re-emission andregardless of the wavelengths involved. Accordingly, the term “phosphor”is used herein to refer to materials that are sometimes calledfluorescent and/or phosphorescent. In general, phosphor particles absorblight having shorter wavelengths and re-emit light having longerwavelengths. As such, some or all of the light emitted by the LED chipat a first wavelength may be absorbed by the phosphor particles, whichmay responsively emit light at a second wavelength. For example, asingle blue emitting LED chip may be surrounded with a yellow phosphor,such as cerium-doped yttrium aluminum garnet (YAG). The resulting light,which is a combination of blue light and yellow light, may appear whiteto an observer.

Typically, phosphor particles are randomly distributed within the matrixof encapsulant material. Some or all of the light emitted by the LEDchip at a first wavelength may be absorbed by the phosphor particles,which may responsively emit light at a second wavelength. For example, ablue-emitting chip may be encapsulated with an encapsulant matrixincluding a yellow-emitting phosphor. The combination of blue light(from the chip) with yellow light (from the phosphor) may produce alight that appears white. Some red-emitting phosphor particles may beincluded in the encapsulant matrix to improve the color renderingproperties of the light, i.e. to make the light appear more “warm.”Similarly, a UV-emitting chip may be encapsulated with an encapsulantmaterial including phosphor particles that individually emit red, greenand blue light upon excitation by UV light. The resulting light, whichis a combination of red, green and blue light, may appear white and mayhave good color rendering properties.

It is important to control the junction temperature of phosphorconverted LEDs in order to provide a long life for LEDs. In conventionalLEDs, the distribution of phosphor particles in a coating on an LED chipis essentially uniform. FIG. 2 illustrates a conventional LED 20 thatincludes a chip 12 and a coating of phosphor-containing material 22,such as silicone, on the light emitting surface 12 a of the chip 12.Because of the generally uniform distribution of the phosphor particlesP in the silicone matrix 22, the average distance that heat generated bythe Stokes loss has to travel is approximately half the coatingthickness, for example 25-50 μm, depending on the target color point.Arrows A₁ graphically illustrate the different distances that heattravels from the various phosphor particles P to the chip 12. Due to thelow thermal conductivity of the silicone matrix 22, heat from thedown-conversion event may not be easily dissipated, causing the phosphorparticles P to heat up and reduce their efficiency. Self-heating of thephosphor particles P, along with poor heat conduction through thecoating material 22, causes LED chip efficiency to drop with increasingflux density, as illustrated in FIG. 3. Moreover, the effects becomemore pronounced as the thickness of the phosphor-containing material 22increases.

SUMMARY

It should be appreciated that this Summary is provided to introduce aselection of concepts in a simplified form, the concepts being furtherdescribed below in the Detailed Description. This Summary is notintended to identify key features or essential features of thisdisclosure, nor is it intended to limit the scope of the invention.

Embodiments of the present invention reduce phosphor self-heatingeffects by improving heat transfer between phosphor particles and an LEDchip by reducing the average separation between phosphor particles andthe chip surface, and by reducing the regions of direct physical contactbetween phosphor particles and the chip surface.

In some embodiments of the present invention, an LED includes a lightemitting element (i.e., an LED chip) having a light emitting surface,and a coating of phosphor-containing material (e.g., silicone) on thelight emitting surface. The phosphor particles are arranged in a denselypacked layer within the coating at the light emitting surface, and suchthat the light emitting surface is in contacting relationship with thelayer of phosphor particles. The light emitting surface emits lighthaving a first dominant wavelength upon the application of a voltage tothe LED chip. The phosphor particles convert light emitted by the lightemitting surface to light having a second dominant wavelength differentfrom the first dominant wavelength.

Because the densely packed layer of phosphor particles is located at thelight emitting surface, coating material above the densely packed layerof phosphor particles is devoid of phosphor particles, and can beremoved and/or reduced, if desired. Also, because the densely packedlayer of phosphor particles is in contacting relationship with the lightemitting surface, heat transfer between the phosphor particles and theLED chip is substantially improved over conventional LEDs. For example,an average distance that heat generated by the phosphor particlestravels from the densely packed layer to the LED chip is substantiallyless than about half a thickness of the coating.

In some embodiments, the light emitting surface may include a pattern offeatures formed therein that extend outward to form receptacles forreceiving phosphor particles. These receptacles provide more surfacearea for phosphor particles to contact, thereby further enhancing heattransfer from the phosphor particles.

In other embodiments, an intermediate layer of material (e.g., siliconnitride, etc.) may be placed between the light emitting surface andcoating of phosphor-containing material. The intermediate layer ofmaterial includes a pattern of features formed therein that extendoutward to form receptacles for receiving phosphor particles. Asdescribed above, these receptacles provide more surface area for thephosphor particles to contact, thereby further enhancing heat transferfrom the phosphor particles.

According to some embodiments of the present invention, a method ofmaking a semiconductor light emitting device includes applying aphosphor-containing material (e.g., silicone, silicon, or any opticalgrade indexing matching carrier) on at least a portion of a lightemitting surface of a chip, causing phosphor particles in the materialto become arranged in a densely packed layer within the material at thelight emitting surface, and curing the material without disturbing thedensely packed layer of phosphor particles. In some embodiments, curedmaterial above the densely packed layer of phosphor particles that isdevoid of phosphor particles can be removed and/or reduced.

In some embodiments, causing the phosphor particles in the material tobecome arranged in a densely packed layer at the light emitting surfaceincludes subjecting the chip to centrifugal force. In some embodiments,causing the phosphor particles in the material to become arranged in adensely packed layer at the light emitting surface includes subjectingthe phosphor-containing material to at least one harmonic vibration, forexample, via a vibration table, or via ultrasonic vibration, etc.

In other embodiments, causing the phosphor particles to become arrangedin a densely packed layer at the light emitting surface includes heatingthe material to a predetermined temperature for a predetermined time tolower the viscosity of the material such that the phosphor particles cansettle under the force of gravity. In some embodiments, the material maybe directly heated. In some embodiments, the material may be directlyheated and/or indirectly heated via the chip.

In other embodiments, the viscosity of the phosphor-containing materialcan be lowered via the addition of a solvent, such as hexane or xylene.The solvent reduces viscosity by breaking the polymer chains (e.g.,silicone chains) of the phosphor-containing material, causing thephosphor particles to drop out. The solvent is subsequently removed, forexample, via evaporation prior to curing of the phosphor-containingmaterial. The polymer chains become re-established upon removal of thesolvent.

In some embodiments, a pattern of features is formed in the lightemitting surface prior to applying the phosphor-containing material. Asdescribed above, the features extend outward to form receptacles forreceiving the phosphor particles.

In other embodiments, an intermediate layer of material (e.g., siliconnitride, etc.) is applied to the light emitting surface prior toapplying the phosphor containing material. The intermediate layer ofmaterial includes a pattern of features extending outward therefrom thatform receptacles for receiving the phosphor particles. Thephosphor-containing material is applied to the intermediate layer ofmaterial.

According to other embodiments of the present invention, a method ofmaking a semiconductor light emitting device includes applying aphosphor-containing carrier material on at least a portion of a lightemitting surface of an LED chip. To obtain a uniform distribution ofphosphor particles across the light emitting surface, the chip may beagitated in some manner. The phosphor particles in the carrier materialare then caused to become arranged in a densely packed layer at thelight emitting surface, and such that the light emitting surface is incontacting relationship with the layer of phosphor particles. Forexample, in some embodiments, causing the phosphor particles in thematerial to become arranged in a densely packed layer at the lightemitting surface comprises subjecting the chip to centrifugal force. Thecarrier material is removed without disturbing the densely packed layerof phosphor particles, and a layer of encapsulating material (e.g.,silicone) is applied to the densely packed layer of phosphor particles,also without disturbing the densely packed layer of phosphor particles.The encapsulating material is then cured without disturbing the denselypacked layer of phosphor particles.

In some embodiments, a pattern of features is formed in the lightemitting surface prior to applying the phosphor-containing carriermaterial. The outwardly extending features form receptacles forreceiving the phosphor particles, as described above. In otherembodiments, an intermediate layer of material (e.g., silicon nitride,etc.) is applied to the light emitting surface prior to applying thephosphor-containing carrier material. The intermediate layer of materialincludes a pattern of features extending outward therefrom that formreceptacles for receiving the phosphor particles.

According to some embodiments of the present invention, a method ofmaking a semiconductor light emitting device includes sequentiallyapplying first, second and third quantities of phosphor particles to atleast a portion of a light emitting surface of a semiconductor lightemitting element. The phosphor particles in the second quantity arelarger than the phosphor particles in both the first and thirdquantities. The smaller phosphor particles in the first and thirdquantities become closely arranged around the larger phosphor particlesin the second quantity so as to form a densely packed layer at the lightemitting surface. A layer of encapsulating material (e.g., silicone) isapplied to the densely packed layer of phosphor particles and then curedwithout disturbing the densely packed layer of phosphor particles.

In some embodiments, the first, second and third quantities of phosphorparticles are sequentially applied to at least a portion of the lightemitting surface through a liquid material, such as silicone, whereinthe viscosity of the liquid material has been reduced. In someembodiments, the viscosity of the liquid material may be reduce byheating the liquid material. In some embodiments, the viscosity of theliquid material is reduced by a solvent, such as xylene or hexane.

According to some embodiments of the present invention, a method ofmaking a semiconductor light emitting device includes applying an amountof phosphor-containing material on at least a portion of a lightemitting surface of a semiconductor light emitting element, and causingphosphor particles in the material to become arranged in a denselypacked layer within the material at the light emitting surface when theamount of phosphor-containing material is sufficient to convert light toa desired color point. The material is then cured without disturbing thedensely packed layer of phosphor particles. In some embodiments, lightconversion by the phosphor-containing material is measured substantiallyin real time as the phosphor-containing material is being applied to thelight emitting surface. The phosphor particles are caused to becomearranged in a densely packed layer at the light emitting surface byapplying a solvent to the phosphor-containing material. The solvent isremoved prior to curing.

According to some embodiments of the present invention, a lightingdevice (e.g., a component, a module, a self-ballasted lamp or lightfixture, etc.) includes an LED chip that emits light having a firstdominant wavelength upon the application of a voltage thereto, and acoating of phosphor-containing material on a light emitting surface ofthe chip. The phosphor particles are arranged in a densely packed layerwithin the material at the light emitting surface such that the lightemitting surface is in contacting relationship with the layer ofphosphor particles. The phosphor particles convert light emitted by thelight emitting surface to light having a second dominant wavelengthdifferent from the first dominant wavelength. Because the densely packedlayer of phosphor particles is in contacting relationship with the lightemitting surface, heat transfer between the phosphor particles and thechip is substantially improved. For example, an average distance thatheat generated by the phosphor particles travels from the densely packedlayer to the chip is substantially less than about half a thickness ofthe coating.

According to some embodiments of the present invention, a semiconductorlight emitting device includes a semiconductor light emitting elementhaving a light emitting surface, an element in adjacent, spacedspaced-apart relationship with the light emitting surface, and a coatingof phosphor-containing material on the element, wherein phosphorparticles are arranged in a densely packed layer within the coating. Insome embodiments, the spaced-apart element is a lens. In someembodiments, the spaced-apart element is reflective element. In someembodiments, the spaced-apart element is a combination of a lens andreflective element. In some embodiments, the spaced-apart element is alayer of material.

It is noted that aspects of the invention described with respect to oneembodiment may be incorporated in a different embodiment although notspecifically described relative thereto. That is, all embodiments and/orfeatures of any embodiment can be combined in any way and/orcombination. Applicant reserves the right to change any originally filedclaim or file any new claim accordingly, including the right to be ableto amend any originally filed claim to depend from and/or incorporateany feature of any other claim although not originally claimed in thatmanner. These and other objects and/or aspects of the present inventionare explained in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate certain embodiment(s) of theinvention.

FIG. 1 is a cross-sectional side view illustrating a conventionalpackaged LED.

FIG. 2 is a cross sectional view of an LED illustrating a conventionalphosphor-containing material applied to a light emitting surface of anLED chip.

FIG. 3 is a graph illustrating heat transfer characteristics of thelight emitting device of FIG. 2.

FIG. 4A is a cross sectional view of an LED chip immediately after aphosphor-containing material has been applied to the light emittingsurface, according to some embodiments of the present invention.

FIG. 4B illustrates the light emitting device of FIG. 4A after phosphorparticles have formed a densely packed layer at the light emittingsurface, according to some embodiments of the present invention.

FIG. 5A is a cross sectional view of an LED chip having a pattern offeatures extending outwardly from the light emitting surface immediatelyafter a phosphor-containing material has been applied to the lightemitting surface, according to some embodiments of the presentinvention.

FIG. 5B illustrates the light emitting device of FIG. 5A after phosphorparticles have formed a densely packed layer at the light emittingsurface and are in contacting relationship with the receptacles,according to some embodiments of the present invention.

FIGS. 6-11 are flow charts of operations for making LEDs, according toembodiments of the present invention.

DETAILED DESCRIPTION

The present invention now will be described more fully with reference tothe accompanying drawings, in which embodiments of the invention areshown. This invention may, however, be embodied in many different formsand should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art. In the drawings, the size andrelative sizes of layers and regions may be exaggerated for clarity.Like numbers refer to like elements throughout.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. It will be understood that if part of an element, such as asurface, is referred to as “inner,” it is farther from the outside ofthe device than other parts of the element. Furthermore, relative termssuch as “beneath” or “overlies” may be used herein to describe arelationship of one layer or region to another layer or region relativeto a substrate or base layer as illustrated in the figures. It will beunderstood that these terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe figures. Finally, the term “directly” means that there are nointervening elements. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother region, layer or section. Thus, a first element, component,region, layer or section discussed below could be termed a secondelement, component, region, layer or section without departing from theteachings of the present invention.

The term “densely packed”, as used herein means that a first layer orstratum contains a high concentration of phosphor particles and a secondlayer or stratum above the first layer contains a substantially lowerconcentration of phosphor particles than the first layer/stratum. Theterm “densely packed” also refers to any particle packing that has aparticle density greater than that which would occur naturally or if thetechnique applied was not used. For example, if a layer of phosphorparticles typically has a thickness of “X”, causing the particles tobecome densely packed in accordance with embodiments of the presentinvention would result in a thickness of “<X”.

Embodiments of the invention are described herein with reference tocross-sectional, perspective, and/or plan view illustrations that areschematic illustrations of idealized embodiments of the invention. Assuch, variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, embodiments of the invention should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result, for example, frommanufacturing. For example, a region illustrated or described as arectangle will, typically, have rounded or curved features due to normalmanufacturing tolerances. Thus, the regions illustrated in the figuresare schematic in nature and their shapes are not intended to illustratethe precise shape of a region of a device and are not intended to limitthe scope of the invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andthis specification and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

Embodiments of the present invention LED chip structures that reduce theaverage distance that heat generated by phosphor particles has to travelby causing the phosphor particles to settle in a dense layer immediatelyabove a chip surface. Heat flow is thereby improved so that the phosphortemperature at a given flux density is reduced, thus improving thedevice efficiency, especially at high flux densities. In someembodiments, surface features are added to a chip which increase thedirect contact area between the phosphor particles and chip. Heat flowis further improved as a result. Embodiments of the present inventionare particularly advantageous because, in addition to improvements inefficiency under normal operating conditions (i.e., steady state),settling a predetermined amount of phosphor enables a specific colorpoint to be met.

Referring now to FIGS. 4A-4B, an LED 20, according to embodiments of thepresent invention, includes a semiconductor light emitting element(i.e., chip) 12 having a light emitting surface 12 a and a coating ofphosphor-containing material 22, such as transparent silicone, on thelight emitting surface 12 a. The light emitting surface 12 a emits lighthaving a first dominant wavelength upon the application of a voltage tothe chip 12. The phosphor particles P in the coating material 22 convertlight emitted by the light emitting surface 12 a to light having asecond dominant wavelength different from the first dominant wavelength.As used herein, “light” refers to any radiation, visible and/orinvisible (such as ultraviolet) that is emitted by an LED chip.Moreover, as used herein, the term “transparent” means that at leastsome optical radiation that enters the coating of phosphor-containingmaterial 22 is emitted from the coating of phosphor-containing material22.

Phosphor particles utilized in embodiments of the present invention mayinclude Cerium-doped Yttrium Aluminum Garnet (YAG) and/or otherconventional phosphors. The phosphor particles may be mixed into a pasteor solution of transparent material comprising silicone usingconventional mixing techniques, to thereby provide thephosphor-containing material 22. In some embodiments, thephosphor-containing material 22 may include a binder, such as an epoxy,a silicon-based matrix and/or other solvent. In some embodiments, thephosphor is configured to convert at least some light that is emittedfrom the light emitting surface 12 a such that light that emerges fromthe LED 20 appears as white light. The phosphor-containing material 22may be applied to the light emitting surface 12 in various waysincluding, but not limited to, screen printing, evaporation (sputter,e-beam, thermal, CVD, electrostatic and/or electropheoric deposition),dipping, spin coating and/or other techniques. The thickness of thephosphor-containing material 22 on the light emitting surface 12 a mayrange between about 2 μm and about 100 μm, in some embodiments of theinvention. However, other thicknesses may be used. The thickness that isused may be selected to reduce or minimize self-absorption and/orscattering and may depend on the coating process, the density of thephosphor and/or the desired application.

The chip 12 may be a light emitting diode, a laser diode and/or othersemiconductor device that includes one or more semiconductor layers,which may include silicon, silicon carbide, gallium nitride and/or othersemiconductor materials, a substrate which may include sapphire,silicon, silicon carbide and/or other microelectronic substrates, andone or more contact layers, which may include metal and/or otherconductive layers. In some embodiments, ultraviolet, blue and/or greenLEDs may be provided. The design and fabrication of LEDs are well knownto those having skill in the art and need not be described in detailherein.

LEDs, according to some embodiments of the present invention, mayinclude structures such as the gallium nitride-based LED and/or laserstructures fabricated on a silicon carbide substrate such as thosedevices manufactured and sold by Cree, Inc. of Durham, N.C. The presentinvention may be suitable for use with LED and/or laser structures asdescribed in U.S. Pat. Nos. 6,201,262; 6,187,606; 6,120,600; 5,912,477;5,739,554; 5,631,190; 5,604,135; 5,523,589; 5,416,342; 5,393,993;5,338,944; 5,210,051; 5,027,168; 5,027,168; 4,966,862 and/or 4,918,497,assigned to the assignee of the present invention, the disclosures ofwhich are incorporated herein by reference in their entirety as if setforth fully herein. Other suitable LED and/or laser structures aredescribed in published United States Patent Application Publication No.US 2003/0006418 A1 entitled Group III Nitride Based Light Emitting DiodeStructures With a Quantum Well and Superlattice, Group III Nitride BasedQuantum Well Structures and Group III Nitride Based SuperlatticeStructures, published Jan. 9, 2003, as well as published United StatesPatent Application Publication No. US 2002/0123164 A1 entitled LightEmitting Diodes Including Modifications for Light Extraction andManufacturing Methods Therefor, both assigned to the assignee of thepresent invention, the disclosures of both of which are herebyincorporated herein by reference in their entirety as if set forth fullyherein. Furthermore, phosphor coated LEDs, such as those described inU.S. Patent Publication No. 2004/0056260 A1, entitled Phosphor-CoatedLight Emitting Diodes Including Tapered Sidewalls and FabricationMethods Therefor, the disclosure of which is incorporated by referenceherein as if set forth fully, may also be suitable for use inembodiments of the present invention.

FIG. 4A illustrates an LED 20 directly after a phosphor-containingmaterial 22 has been applied to the light emitting surface 12 a of thechip 12. As illustrated, the phosphor particles P are substantiallyuniformly distributed within the coating material 22. FIG. 4Billustrates the LED 20 after the phosphor particles P in the material 22have settled at the light emitting surface 12 a and become arranged in adensely packed layer L, according to embodiments of the presentinvention. The densely packed layer L of phosphor particles Pfacilitates the transfer of heat from the phosphor particles P to thechip 12. Moreover, the location of the densely packed layer L at thelight emitting surface 12 a decreases the average distance heatgenerated by the phosphor particles P travels to the chip 12. Arrows A₂graphically illustrate that the distance heat travels from the phosphorparticles P to the chip 12 is substantially the same among the phosphorparticles P, and the average distance of travel is substantially reducedcompared with arrows A₁ of FIG. 2. For example, the average distancethat heat generated by the phosphor particles P travels from the denselypacked layer L to the chip 12 is substantially less than half thethickness of the coating 22.

Because the densely packed layer L of phosphor particles P is located atthe light emitting surface 12 a, coating material 22 above the denselypacked layer L of phosphor particles P is devoid of phosphor particlesP, and can be removed and/or reduced, if desired. The portion of thecoating material 22 that can be removed and/or reduced is represented inFIG. 4B via a dotted line.

The phosphor particles P can be encouraged to become arranged in adensely packed layer L at the light emitting surface 12 a in variousways. In some embodiments, the chip 12 and phosphor-containing coatingmaterial 22 can be subjected to centrifugal force, for example, via acentrifuge. In other embodiments, causing the phosphor particles P tobecome arranged in a densely packed layer L at the light emittingsurface 12 a includes heating the coating material 22 to a predeterminedtemperature and for a predetermined time to lower the viscosity of thecoating material 22 such that the phosphor particles P can settle at thelight emitting surface 12 a prior to curing the coating material 22.

The amount of phosphor in the coating material 22 and the area coveredby the phosphor-containing coating material 22 may be selected toprovide a desired light output. The selection may be made in advance ormay be tuned when the LED 20 is constructed. The phosphor-containingmaterial 22 may be applied using any suitable phosphor depositiontechnique, such as by inkjet or bubble jet printing, screen depositionor other techniques.

Suitable red phosphors for embodiments of the present invention include,but are not limited to, Sr2Si5N8:Eu2+, and CaAlSiN3:Eu. Other redphosphors that can be used include, but are not limited to, phosphorsfrom the Eu2+-SiAlON family of phosphors, as well as CaSiN2:Ce3+,CaSiN2:Eu2+ and/or phosphors from the (Ca,Si,Ba)SiO4:Eu2+ (BOSE) family.Suitable yellow phosphors include, but are not limited to, Y3Al5O12:Ce3+(Ce:YAG), CaAlSiN3:Ce3+, and phosphors from the Eu2+-SiAlON-family,and/or the BOSE family. Suitable green phosphors include, but are notlimited to, phosphors from the BOSE family, as well as CaSi2O2N2:Eu2+.The phosphor may also be doped at any suitable level to provide adesired wavelength of light output. In some embodiments, Ce and/or Eumay be doped into a phosphor at a dopant concentration in a range ofabout 0.1% to about 20%. Suitable phosphors are available from numeroussuppliers, including Mitsubishi Chemical Corporation, Tokyo, Japan,Leuchtstoffwerk Breitungen GmbH, Breitungen, Germany, and IntematixCompany, Fremont, Calif.

Other suitable phosphors that may be utilized according to embodimentsof the present invention include nanocrystals (NCs) with a cadmiumselenide (CdSe) core surrounded by a zinc sulfide (ZnS) shell that canconvert short wavelengths to longer ones. Such crystals can effectivelyabsorb UV-green light and emit green-red light. The absorption andemission spectra of these NCs can be tuned by controlling the diameterof the CdSe core and the thickness of the ZnS shell. Moreover, the NCshave the advantages of high quantum efficiency and photostability. Inparticular, mixing the NCs with gold nanoparticles induces couplingbetween CdSe/ZnS NCs and surface plasmons (SPs) on the gold that canenhance the color conversion efficiency.

Referring now to FIGS. 5A-5B, an LED 20, according to other embodimentsof the present invention, has a light emitting surface that includes apattern of features 12 f formed therein that extend outward to formreceptacles R for receiving phosphor particles P. These receptacles Rprovide more surface area for the phosphor particles P to contact,thereby further enhancing heat transfer from the phosphor particles tothe chip 12. The pattern of features 12 f may be formed directly in anepitaxial layer of the light emitting element 12 or another intermediarylayer (not shown), such as silicon nitride, may be provided with apattern of features. In contrast with a planar light emitting surface,such as illustrated in FIGS. 4A-4B, wherein phosphor particles can onlymake single contact with a light emitting surface, the embodiment ofFIGS. 5A-5B allows phosphor particles P to touch the light emittingsurface 12 a at multiple points. This is advantageous because themultiple contact points enhances heat transfer from the phosphorparticles P to the chip 12. Arrows A₃ illustrate the multiple contactpoints of a phosphor particle P and, thus, the multiple heat transferpaths for each phosphor particle P.

Operations for making LEDs, according to embodiments of the presentinvention, are described with reference to FIGS. 6-9. Referringinitially to FIG. 6, in some embodiments, a pattern of features may beformed in a light emitting surface of an LED chip so as to extendoutward therefrom and form receptacles configured to receive phosphorparticles (Block 100). In other embodiments, an intermediate layer ofmaterial, such as silicon nitride, may be positioned on a light emittingsurface of a chip (Block 110). The intermediate layer of material mayinclude a pattern of features that extend outward and form receptaclesconfigured to receive phosphor particles. A coating ofphosphor-containing material is applied to the light emitting surface orto an intermediate layer on the light emitting surface (Block 120).

The phosphor particles in the coating material are caused to becomearranged in a densely packed layer at the light emitting surface, orsurface of an intermediate layer, if present (Block 130). The formationof the densely packed layer may be facilitated by subjecting the lightemitting element to centrifugal forces, such as via a centrifuge (Block132, FIG. 7) and/or by heating the coating material sufficiently toreduce viscosity such that the phosphor particles can settle to the chipsurface (Block 134, FIG. 7) and/or by adding a solvent to the coatingmaterial (Block 136, FIG. 7). As would be understood by those of skillin the art, spinning via a centrifuge would move phosphor particles tooutside (against chip surface). In some embodiments, causing thephosphor particles to become arranged in a densely packed layer at thelight emitting surface includes subjecting the phosphor-containingmaterial to at least one harmonic vibration, for example, via avibration table (Block 138, FIG. 7). The coating material is then curedwithout disturbing the densely packed layer of particles (Block 140).For example, the coating material 22 may be cured at between about 50°C. and about 200° C. for about several seconds to several hours. In someembodiments, cured coating material above the densely packed layer thatis devoid of phosphor particles is removed and/or reduced. (Block 150).

Referring to FIG. 8, operations for making LEDs, according to otherembodiments of the present invention, are described. In someembodiments, a pattern of features may be formed in a light emittingsurface of an LED chip so as to extend outward from the light emittingsurface and form receptacles configured to receive phosphor particles(Block 200). In other embodiments, an intermediate layer of material,such as silicon nitride, may be positioned on a light emitting surfaceof a chip (Block 210). The intermediate layer of material may include apattern of features that extend outward and form receptacles configuredto receive phosphor particles. A phosphor-containing carrier material isthen applied to the light emitting surface or to an intermediate layeron the light emitting surface (Block 220).

The light emitting element may be agitated in some manner to obtainuniform distribution of phosphor particles across the light emittingsurface, or surface of an intermediate layer, if present (Block 230).For example, agitation may be performed as described in “Self-Assemblyof Particles for Densest Packing By Mechanical Vibration”, A. B. Yu, etal., Phys Rev Lett, December 2006, which is incorporated herein byreference in its entirety. However, various other methods of agitationmay be utilized, without limitation. The phosphor particles in thecarrier material are caused to become arranged in a densely packed layerat the light emitting surface, or surface of an intermediate layer, ifpresent (Block 240). The formation of the densely packed layer may befacilitated by subjecting the chip to centrifugal force (Block 242, FIG.9).

The carrier material is then removed without disturbing the denselypacked layer of phosphor particles (Block 250) and a layer ofencapsulating material is applied over the densely packed layer, alsowithout disturbing the densely packed layer of phosphor particles (Block260). In some embodiments, no additional encapsulating material isapplied. The encapsulating material is then cured without disturbing thedensely packed layer of particles (Block 270). In some embodiments,cured encapsulating material above the densely packed layer that isdevoid of phosphor particles is removed and/or reduced. (Block 280).

Referring to FIG. 10, operations for making LEDs, according to otherembodiments of the present invention, are described. In someembodiments, first, second and third quantities of phosphor particlesare sequentially applied to at least a portion of a light emittingsurface of a semiconductor light emitting element (Block 300). Phosphorparticles in the first and third quantities are small in size relativeto phosphor particles in the second quantity. As such, the smallerphosphor particles in the first and third quantities become closelyarranged around the larger phosphor particles in the second quantity soas to form a densely packed layer at the light emitting surface. A layerof encapsulating material (e.g., silicone) is applied to the denselypacked layer of phosphor particles (Block 302) and then cured (Block304) without disturbing the densely packed layer of phosphor particles.

The first, second and third quantities of phosphor particles aresequentially applied in a reduced viscosity environment. Such anenvironment may include air or a liquid polymeric material, such assilicone, that has its viscosity lowered. If a liquid material used, theviscosity can be lowered by heating the liquid material and/or by addinga solvent, such as xylene or hexane. The solvent reduces viscosity ofthe liquid material by breaking the polymer chains (e.g., siliconechains) of the phosphor-containing material. This causes the phosphorparticles to drop out to the light emitting surface. The solvent issubsequently removed, for example, via evaporation prior to curing ofthe phosphor-containing material. Upon removal of the solvent, thepolymer chains become re-established.

Referring to FIG. 11, operations for making LEDs, according to otherembodiments of the present invention, are described. In someembodiments, phosphor-containing material is applied on at least aportion of a light emitting surface of a semiconductor light emittingelement (Block 400). The light emitting surface emits light having afirst dominant wavelength upon the application of a voltage to thesemiconductor light emitting element, and the phosphor particles in thephosphor-containing material convert light emitted by the light emittingsurface to light having a second dominant wavelength different from thefirst dominant wavelength. The amount of light conversion by thephosphor particles in the phosphor-containing material is measured(Block 402). In some embodiments, the amount of light conversion by thephosphor particles in the phosphor-containing material is measured inreal time as the phosphor-containing material is applied to the lightemitting surface. In other embodiments, an amount is added and thenlight conversion measurements are taken. This iteration is repeateduntil a desired color point is reached.

When the amount of phosphor-containing material is sufficient to convertlight to a desired color point, no more phosphor-containing material isadded to the light emitting surface (Block 404). The phosphor particlesin the material are then caused to become arranged in a densely packedlayer within the material at the light emitting surface (Block 406). Asdescribed above, this may be effected by heating the phosphor-containingmaterial to lower its viscosity and/or via the addition of a solventthat also lowers the viscosity of the phosphor-containing material. Thematerial is then cured without disturbing the densely packed layer ofphosphor particles (Block 408). If a solvent was used to lowerviscosity, the solvent is removed, for example, via evaporation, priorto curing.

LEDs according to embodiments of the present invention may be utilizedin light fixtures. As used herein, the term light fixture includesdevices that are classified under UL 1598 safety standard. Exemplarylight fixtures include, but are not limited to, lay-in fixtures, such asthe LR24 from Cree LED Lighting, downlights, wall washer, pendants,surface mounts, and the like.

While particular embodiments are described herein, various combinationsand sub-combinations of the structures described herein are contemplatedand will be apparent to a skilled person having knowledge of thisdisclosure.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. Although a few exemplary embodiments ofthis invention have been described, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe claims. Therefore, it is to be understood that the foregoing isillustrative of the present invention and is not to be construed aslimited to the specific embodiments disclosed, and that modifications tothe disclosed embodiments, as well as other embodiments, are intended tobe included within the scope of the appended claims. The invention isdefined by the following claims, with equivalents of the claims to beincluded therein.

What is claimed is:
 1. A semiconductor light emitting device, comprising: a semiconductor light emitting element having a light emitting surface; and a coating of phosphor-containing material on the light emitting surface, wherein phosphor particles are arranged in a densely packed layer within the coating at the light emitting surface, and such that the light emitting surface is in contacting relationship with the densely packed layer of phosphor particles, and wherein the densely packed layer of phosphor particles does not extend all the way through the coating.
 2. The semiconductor light emitting device of claim 1, wherein an average distance that heat generated by the phosphor particles travels from the densely packed layer to the semiconductor light emitting element is substantially less than half the thickness of the coating.
 3. The semiconductor light emitting device of claim 1, wherein the light emitting surface comprises a pattern of features extending outwardly to form receptacles for receiving the phosphor particles.
 4. The semiconductor light emitting device of claim 1, wherein the light emitting surface emits light having a first dominant wavelength upon the application of a voltage to the semiconductor light emitting element, and wherein the phosphor particles convert light emitted by the light emitting surface to light having a second dominant wavelength different from the first dominant wavelength.
 5. The semiconductor light emitting device of claim 1, wherein the semiconductor light emitting element is selected from the group consisting of light emitting diodes and laser diodes.
 6. The semiconductor light emitting device of claim 1, wherein the coating of phosphor-containing material comprises silicone or silicon.
 7. A lighting device, comprising: a semiconductor light emitting element that emits light having a first dominant wavelength upon the application of a voltage thereto; and a coating of phosphor-containing material on a light emitting surface of the element, wherein phosphor particles are arranged in a densely packed layer within the material at the light emitting surface, and such that the light emitting surface is in contacting relationship with the densely packed layer of phosphor particles, wherein the phosphor particles convert light emitted by the light emitting surface to light having a second dominant wavelength different from the first dominant wavelength, and wherein the densely packed layer of phosphor particles does not extend all the way through the coating.
 8. The lighting device of claim 7, wherein the light emitting surface comprises a pattern of features extending outward from the light emitting surface that form receptacles for receiving the phosphor particles.
 9. A semiconductor light emitting device, comprising: a semiconductor light emitting element having a light emitting surface; an element in adjacent, spaced-apart relationship with the light emitting surface; and a coating of phosphor-containing material on the element, wherein phosphor particles are arranged in a densely packed layer within the coating.
 10. The semiconductor light emitting device of claim 9, wherein the element comprises a lens.
 11. The semiconductor light emitting device of claim 9, wherein the element comprises a reflective element.
 12. A semiconductor light emitting device, comprising: a semiconductor light emitting element having a light emitting surface wherein the light emitting surface comprises a pattern of features extending outwardly to form receptacles; and a coating of phosphor-containing material on the light emitting surface, wherein phosphor particles are arranged in a densely packed layer within the coating at the light emitting surface, and such that phosphor particles in the densely packed layer are received within the light emitting surface receptacles, and wherein the densely packed layer of phosphor particles does not extend all the way through the coating.
 13. The semiconductor light emitting device of claim 12, wherein an average distance that heat generated by the phosphor particles travels from the densely packed layer to the semiconductor light emitting element is substantially less than half the thickness of the coating.
 14. The semiconductor light emitting device of claim 12, wherein the light emitting surface emits light having a first dominant wavelength upon the application of a voltage to the semiconductor light emitting element, and wherein the phosphor particles convert light emitted by the light emitting surface to light having a second dominant wavelength different from the first dominant wavelength.
 15. The semiconductor light emitting device of claim 12, wherein the semiconductor light emitting element is selected from the group consisting of light emitting diodes and laser diodes.
 16. The semiconductor light emitting device of claim 12, wherein the coating of phosphor-containing material comprises silicone or silicon.
 17. A semiconductor light emitting device, comprising: a semiconductor light emitting element having a light emitting surface; an intermediate layer of material on the light emitting surface, wherein the intermediate layer of material comprises a pattern of features extending outwardly therefrom to form receptacles for receiving phosphor particles; and a coating of phosphor-containing material on the intermediate layer of material, wherein phosphor particles are arranged in a densely packed layer within the coating at the intermediate layer of material and such that phosphor particles in the densely packed layer are received within the receptacles, and wherein the densely packed layer of phosphor particles does not extend all the way through the coating.
 18. The semiconductor light emitting device of claim 17, wherein an average distance that heat generated by the phosphor particles travels from the densely packed layer to the semiconductor light emitting element is substantially less than half the thickness of the coating.
 19. The semiconductor light emitting device of claim 17, wherein the light emitting surface emits light having a first dominant wavelength upon the application of a voltage to the semiconductor light emitting element, and wherein the phosphor particles convert light emitted by the light emitting surface to light having a second dominant wavelength different from the first dominant wavelength.
 20. The semiconductor light emitting device of claim 17, wherein the semiconductor light emitting element is selected from the group consisting of light emitting diodes and laser diodes.
 21. The semiconductor light emitting device of claim 17, wherein the coating of phosphor-containing material comprises silicone or silicon.
 22. A lighting device, comprising: a semiconductor light emitting element that emits light having a first dominant wavelength upon the application of a voltage thereto; an intermediate layer of material on the light emitting element, wherein the intermediate layer of material comprises a pattern of features extending outwardly therefrom to form receptacles for receiving phosphor particles; and a coating of phosphor-containing material on the intermediate layer of material, wherein phosphor particles are arranged in a densely packed layer within the coating at the intermediate layer of material and such that phosphor particles in the densely packed layer are received within the receptacles, wherein the phosphor particles convert light emitted by the light emitting element to light having a second dominant wavelength different from the first dominant wavelength, and wherein the densely packed layer of phosphor particles does not extend all the way through the coating.
 23. The semiconductor light emitting device of claim 22, wherein an average distance that heat generated by the phosphor particles travels from the densely packed layer to the semiconductor light emitting element is substantially less than half the thickness of the coating.
 24. The semiconductor light emitting device of claim 22, wherein the light emitting surface emits light having a first dominant wavelength upon the application of a voltage to the semiconductor light emitting element, and wherein the phosphor particles convert light emitted by the light emitting surface to light having a second dominant wavelength different from the first dominant wavelength.
 25. The semiconductor light emitting device of claim 22, wherein the semiconductor light emitting element is selected from the group consisting of light emitting diodes and laser diodes.
 26. The semiconductor light emitting device of claim 22, wherein the coating of phosphor-containing material comprises silicone or silicon. 